# <locale.h>

**[<locale.h>](https://en.cppreference.com/w/c/header/locale)** controls culture-specific formatting: decimal separators, currency symbols, sorting order. By default programs run in the `"C"` locale (ASCII, `.` decimal); call `setlocale(LC_ALL, "")` to use the user's environment locale.

Be careful with `LC_NUMERIC`—changing the decimal separator can break parsing of machine-generated data.

## Example

This example demonstrates how locale affects number formatting.

```c
// compile: gcc -o localeexample localeexample.c
// run: ./localeexample
// description: show how locale affects formatting

#include <locale.h>
#include <stdio.h>

int main() {
    double pi = 3.141592;
    
    setlocale(LC_ALL, "C");
    printf("C locale: %f\n", pi);
    
    setlocale(LC_NUMERIC, "de_DE.UTF-8");
    printf("German locale: %f\n", pi);
    
    return 0;
}
```

## Common functions and macros

**Locale selection**:
- `setlocale(category, locale)`: set or query locale
- `localeconv()`: get locale numeric/monetary info (returns `struct lconv*`)

**Categories** (first arg to setlocale):
- `LC_ALL`: all categories
- `LC_COLLATE`: string collation (strcoll, strxfrm)
- `LC_CTYPE`: character classification (isalpha, tolower)
- `LC_MONETARY`: currency formatting
- `LC_NUMERIC`: number/decimal separator (affects printf/scanf)
- `LC_TIME`: date/time formatting

**Common locales**:
- `"C"`: minimal C locale (ASCII, `.` decimal)
- `"en_US.UTF-8"`: US English with UTF-8
- `"de_DE.UTF-8"`: German with UTF-8
- `""`: user's environment locale